This invention relates to thermal radiators, and, more particularly to radiators suitable for use in conjunction with systems for generating power in space.
Presently, growing requirements for space power in the multi-hundred kilowatt range are expected, particularly in light of the success of the space shuttle program, and it is anticipated that such power requirements will be met by solar, chemical (by means of batteries) or nuclear generation of power. Regardless of the method of generation of power, means for rejecting the heat created by the power generation by radiating the heat into free space is likely to be required (In space, the only way heat can be dissipated is through radiation.)
With specific reference to the solar generation of power, the solar arrays used to generate the power in space generally include four major components: a primary structure, which supports and extends the array as a whole; either reflecting or refracting optical concentrators; heat rejection devices; and a solar panel including cells, substrate, covers and harness.
The cost of solar arrays now being designed or utilized run to several hundred dollars per watt. One way of reducing the cost per watt is to concentrate sunlight so as to require fewer solar cells to generate a given electrical output. However, the concentration of sunlight also results in higher cell temperatures. Because the energy conversion efficiency of solar cells is inversely proportional to the temperature of the cells, the concentration of sunlight requires either acceptance of lower cell performance characteristics or the introduction of heat rejection methods, such as radiators.
Radiative heat rejection systems are generally area intensive, and, therefore, the larger and more massive the structure, the greater its rate of heat transfer. However, for space applications, it is desirable to keep the mass of the solar array as low as possible. Thus, typical honeycomb or aluminium pipe structures having a sufficiently high heat transfer rate are likely to be too heavy for space usage.
Because of their long life and modularization capabilities, semi-passive radiators--radiators in which heat pipes carry heat by vapor transport from evaporator sections located near the solar cells to condensation sections attached to extended radiating surfaces--are preferred for space applications, as opposed to active radiators (in which heat transfer fluid is pumped between the cell region and the radiating surface), or passive radiators (in which heat is transported to the radiator solely by conduction). In semi-passive radiators, the cost of the heat pipes dominates the cost of the radiator as a whole due to high fabrication expenses for the pipes, the close tolerances required in assembling the system, and the time-consuming nature of filling the pipes with the heat transfer fluid and then testing the pipes. Further, damage to the heat pipes due to environmental hazards such as micrometeorite impact is particularly critical, as a damaged heat pipe could leak its heat transport fluid. Thus, thermal radiator design for space should minimize damage potential from micrometeorites, a consideration of no concern to non-space thermal radiators.
Another consideration for a space radiator is that the material have superior ionizing radiation resistance, which consideration is not present in the usual non-space radiator. The heat rejection capability of any radiator is also indirectly proportional to its emissivity.